[zese]

Dear all

I have some ambiguities about the drag coefficient greater than 1?

we know that Cd=1 means like having a vertical plane in a flow stream perpendicular to it. However, what Cd >1 physically mean?

the Drag Coeff = Drag Force / Force induced by dynamic pressure. does it make sense to be greater than 1? I know there are many objects with Cd >1 but it is a confusing issue for me.

BR

[Chris2337]

I think a flat plate perpendicular to the freestream actually has a drag coefficient above 1 (something like 1.3), but is said to be 1 to make it easy to relate.

I would say a drag coefficient over 1 just means that thing is really draggy, but by comparison airfoils and wings regularly operate at lift coefficients over 1 and it doesn't really mean much physically. I guess how you're thinking about it would mean the force of drag is greater than the dynamic pressure multiplied by a reference area. The thing is that reference area can be kind of arbitrary, in terms of the underlying physics, so a drag coefficient over 1 doesn't mean much.

[LuckyTran]

If the plate stops the oncoming flow with a cross-sectional area exactly equal to the plate's footprint, its Cd would be 1 as pointed out. However, the plate does even more than this. The flow that would have been stopped must accelerate over the edges of the plate and interacts with the surrounding flow, generating even more blockage, and so on.

For strangely shaped bodies, the reference area can be a bit tricky but the plate is a good example. Drag coefficient greater than 1 means that the area of the streamtube influenced by the body exceeds the reference area. And yes the plate does have a drag coefficient of ~1.3 (1.28 in some correlations).

[zese]

Originally Posted by LuckyTran

If the plate stops the oncoming flow with a cross-sectional area exactly equal to the plate's footprint, its Cd would be 1 as pointed out. However, the plate does even more than this. The flow that would have been stopped must accelerate over the edges of the plate and interacts with the surrounding flow, generating even more blockage, and so on.

For strangely shaped bodies, the reference area can be a bit tricky but the plate is a good example. Drag coefficient greater than 1 means that the area of the streamtube influenced by the body exceeds the reference area. And yes the plate does have a drag coefficient of ~1.3 (1.28 in some correlations).

Thank you LuckyTran!